The maximum allowable working pressure on a standard boiler shall be determined in accordance with the ASME Code under which it was constructed and stamped. The maximum allowable working pressure on the shell of a nonstandard boiler shall be determined by the strength of the weakest section of the structure computed by multiplying TS by t and E and dividing the product by R multiplied by FS: TS = ultimate tensile strength of the shell plate, PSI, which, if unknown, shall be: 55,000 PSI for steel; and 45,000 PSI for wrought iron; t = minimum thickness of the shell plate of the weakest course, in inches; E = efficiency of the longitudinal joint, which is, for: Tube ligaments and riveted construction, established by Section I of the ASME Code for power boilers; and Seamless construction, 100 percent; R = inside radius of the weakest course of the shell, in inches; FS = factor of safety required by this regulation; The resistance to crushing of mild steel is assumed to be 95,000 PSI of cross-sectional area; When computing the ultimate strength of rivets in shear, the following values, in pounds per square inch, of the cross-sectional area of the rivet shank shall be used: Iron rivets in single shear 38,000 PSI; Iron rivets in double shear 76,000 PSI; Steel rivets in single shear 44,000 PSI; and Steel rivets in double shear 88,000 PSI; When the diameter of a rivet hole in a longitudinal joint of a boiler is not known, the diameter and cross-sectional area of the rivet, after driving, shall be: Ascertained by cutting out one rivet in the body of the joint; or As indicated in the following table: The maximum allowable working pressure on a: Water-tube boiler having tubes secured to cast iron or malleable-iron headers, or having cast-iron mud drums, shall be 160 PSIG; Cast-iron boiler, except a hot-water boiler, shall be 15 PSIG; Boiler having lap-riveted longitudinal joints, operated at a pressure greater than 50 PSI, when removed from an existing setting and reinstalled, shall be 15 PSIG; and Reinstalled horizontal-return tubular boiler having a continuous longitudinal lap seam more than 12 feet long, shall be 15 PSIG. Except as otherwise provided by this regulation, the lowest factor of safety shall be, for: An existing installation: 4.5; A horizontal-return tubular boiler having continuous longitudinal lap seams more than 12 feet long: 8; A reinstalled or secondhand boiler having longitudinal seams of lap-riveted construction: 6; and A reinstalled or secondhand boiler with longitudinal seams of butt and double-strap construction: 5.